It is in general inevitable that impurities from a galvanizing apparatus, an electrode, galvanizing bath materials and a steel sheet to be electro-galvanized are entangled into a galvanizing bath during electro-galvanizing operations of the steel sheet. Impurities thus mixed into the galvanizing bath not only causes degradation of the surface quality of the produced galvanizing layer, but also exert adverse effects on a chromate treatment to be applied thereafter. If, for example, a galvanizing bath contains Fe.sup.2.sup.+ mixed in it as impurities, formation of a chromate film on the galvanizing layer of an electro-galvanized steel sheet is seriously impaired in applying a chromate treatment as the next step, and hence, the amount of deposited chromate is largely reduced. If, furthermore, impurities such as Cu.sup.2.sup.+ and Ni.sup.2.sup.+ are contained in a galvanizing bath, the amount of chromate deposited onto the galvanizing layer of an electro-galvanized steel sheet is small in applying a chromate treatment. Consequently, an intensification of chromate treatment conditions, as described later, cannot increase at all the amount of deposited chromate. As a result, it is impossible to obtain a chromated electro-galvanized steel sheet having a satisfactory corrosion resistance.
For the purpose of preventing impurities from coming into a galvanizing bath, or removing impurities from a galvanizing bath, it has been usual practice to apply a closer control over impurities in a galvanizing bath, to employ a corrosion resistant material for the construction of a galvanizing apparatus, to remove such impurities as cadmium, lead and copper dissolved in a galvanizing bath by substituting zinc for such impurities through a treatment of the galvanizing bath with zinc powder, or, to cause precipitation of such impurities as copper by suspending an iron plate in a galvanizing bath.
On the other hand, a method for intensifying chromating conditions is conventionally known, which comprises increasing the amount of deposited chromate by increasing the amount of free acid in a chromating bath, with a view to imparting a satisfactory corrosion resistance to an electro-galvanized steel sheet of which the galvanizing layer has been degraded by impurities in the galvanizing bath. The chromating bath in this method has however a strong pickling action because of its increased free acid. This method is therefore defective in that the formation of a chromate film becomes non-uniform or the increased dissolution of zinc into the chromating bath accelerates the degradation of the chromating bath. Even by such an intensification of chromating conditions, therefore, the time before occurrence of white rust in a salt spray test, for example, is not extended so much, and an improvement of the corrosion resistance of a chromated electro-galvanized steel sheet cannot be expected.
In all cases, these conventional measures to prevent impurities from coming into a galvanizing bath, to remove impurities from a galvanizing bath and to intensify chromating conditions are only passive actions aiming at preventing the adaptability of chromating of an electro-galvanized steel sheet from being impaired by impurities mixed in a galvanizing bath. These measures cannot therefore be positive actions imparting a higher corrosion resistance to an electro-galvanized steel sheet by improving its adaptability to chromating.
In view of the foregoing, the following methods have so far been proposed:
1. Method which comprises electro-galvanizing a steel sheet in a galvanizing bath containing added Mo and W (refer to the Japanese Patent Publication No. 25,245/71);
2. Method which comprises electro-galvanizing a steel sheet in a galvanizing bath containing added Co, Mo, W and Fe (refer to the Japanese patent Publication No. 16,522/72);
3. Method which comprises electro-galvanizing a steel sheet in a galvanizing bath containing added Co, Mo, W, Ni, Sn, Pb and Fe (refer to the Japanese Patent Publication No. 19,979/74);
4. Method which comprises electro-galvanizing a steel sheet in a galvanizing bath containing added 0.05 - 0.3 g/l Cr.sup.6.sup.+ (refer to the Japanese Patent Provisional Publication No. 84,040/73); and
5. Method which comprises electro-galvanizing a steel sheet in a galvanizing bath containing added 0.05 - 1.5 g/l Zr (refer to the Japanese patent Publication No. 18,202/70).
All these methods (1) to (5) have an object to improve the quality of the galvanizing layer itself of an electro-galvanized steel sheet. The adaptability to chromating of an electro-galvanized steel sheet is not therefore improved by any of these methods, thus leading to no improvement in the corrosion resistance of the electro-galvanized steel sheet after a chromate treatment.
The amount of deposited zinc is in general smaller in an electro-galvanized steel sheet than in a hot-dip galvanized steel sheet. Consequently, an electro-galvanized steel sheet has been superior to a hot-dip galvanized steel sheet in terms of the formability, but has inevitably been inferior to the latter in terms of the corrosion resistance of its galvanizing layer itself (hereinafter called the "bare corrosion resistance"). In this respect, a galvanizing layer containing Co, as in methods (2) and (3) above, has certainly an improved bare corrosion resistance, but in contrast, shows a lower adaptability to chromating, hence a smaller amount of deposited chromate, as described above.
In view of the foregoing, a process for manufacturing a chromated electro-galvanized steel sheet has been proposed (refer to the Japanese Patent Provisional Publication No. 102,538/75), with a view to increasing the amount of deposited chromate by the improvement of the adaptability to chromating of an electro-galvanized steel sheet and thus to improving the corrosion resistance of the electro-galvanized steel sheet after chromating, said process comprising: electro-galvanizing a steel sheet in a Zn-ion based acidic galvanizing bath containing an additive selected from the group consisting of:
______________________________________ (a) Cr.sup.3.sup.+ 50 - 700 ppm, (b) Cr.sup.6.sup.+ 50 - 500 ppm, and (c) Cr.sup.3.sup.+ and Cr.sup.6.sup.+ 50 - 700 ppm, in which Cr.sup.6.sup.+ being 500 ppm at the maximum; ______________________________________
and then subjecting said electro-galvanized steel sheet to a chromate treatment. According to this method, the time before occurrence of white rust is largely extended, but sufficiently satisfactory results are not as yet available in terms of the time before occurrence of red rust.
In view of these facts, it has been hoped to have a process for manufacturing a chromated electro-galvanized steel sheet having not only an excellent bare corrosion resistance but also an excellent corrosion resistance after a chromate treatment, but no such process has as yet been proposed.